The manufacture of shaped polymer articles by extrusion of a solution of cellulose in an aqueous tertiary amine N-oxide solvent (which solution may also be called a dope) into an aqueous coagulating bath is described for example in U.S. Pat. No. 4,246,221, the contents of which are incorporated herein by way of reference. Conventional dissolving-grade cellulose, for example in the form of woodpulp and cotton linters, is utilised as raw material in such processes.
EP-A-0,648,808 points out that the extrusion conditions for such solutions must be chosen to provide freedom from melt flow instability in the extrusion orifice, which can result in melt fracture and consequential production breakdown (loss of spinning stability). Melt flow instability can be countered without reducing extrusion productivity by reducing the viscosity of the solution, for example by reducing the concentration or degree of polymerisation (D.P.) of the cellulose in the solution. Such reduction also permits increases in the draft ratio which can be applied to extruded fibres and in the take-up velocity of such fibres. Nevertheless, such reduction is attended by disadvantages elsewhere in the process, notably by reductions in productivity and by increases in the load on the solvent recovery system. EP-A-0,648,808 describes a solution of cellulose in aqueous N-methylmorpholine N-oxide, wherein the cellulose comprises a mixture of (1) a first cellulose component having a degree of polymerisation (D.P.) in the range from 500 to 2000, and (2) a second cellulose component having a D.P. in the range from 350 to 900, with the proviso that the ratio of the D.P. of component (2) to the D.P. of component (1) is no greater than 0.9:1, the ratio by weight of component (1) to component (2) being in the range from 95:5 to 50:50. It is said that such solutions can be extruded at high velocity with process stability to yield lyocell fibres with mechanical properties similar to those produced by conventional lyocell processes.